1. Field of the Invention
The present invention relates to a method for making transparent thermal dye transfer images having an enhanced density and to a receiving sheet for use according to that method.
2. Description of the Prior Art
Transparent receiving sheets are used for making transparencies by thermal dye transfer processes. The carrier of such receiving sheets is made of a transparent film e.g. of polyethylene terephthalate, a polyether sulfone, a polyimide, a cellulose ester, or a polyvinyl alcohol-coacetal. To avoid poor adsorption of the transferred dye to the film carrier the latter must be provided with a special surface, generally known as dye-image-receiving layer, into which the dye can diffuse more readily. This dye-image-recieving layer should also be transparent, of course. The adhesion of the dye-image-receiving layer to the film carrier can be improved by providing a transparent subbing layer in between.
Black-and-white and/or colour transparencies can be made by printing with an adapted dye-donor element. The transparencies can find wide application in such different fields like i.a. the field of graphic arts and the medical diagnostical field.
For the production of colour transparencies use is made of dye-donor elements comprising repeated separate areas of different dyes, which are heated up sequentially in correspondence with the cyan, magenta, yellow, and possibly black electrical signals, so that dye from the selectively heated regions of the dye-donor element is transferred to the transparent receiving sheet and forms a pattern thereon, the shape and density of which are in accordance with the pattern and intensity of the heat supplied to the dye-donor element. Depending on the number of different dye areas used, 3 or 4 passages are necessary to print the different dyes in register.
For the production of monochromic transparencies use is made of dye-donor elements that have but one dye area. For the production of black-and-white transparencies use is made of dye-donor elements having a black dye area. Instead of a black dye a mixture of dyes can also be employed, which mixture is then chosen such that a neutral black transfer image is obtained. It is of course also possible to produce a black image by printing from several dye areas one dye over the other and in register. However, this procedure is less suitable because it is more time-consuming and needs a higher length of donor element.
The transmission density of transparencies produced hitherto according to known thermal dye transfer methods is rather low and in most of the commercial systems--in spite of the use of donor elements specially designed for printing transparencies--only reaches 1 to 1.2 (as measured by a Macbeth Quantalog Densitometer Type TD 102). However, for many application fields a considerably higher transmission density is asked for. For instance in the medical diagnostical field such as video imaging a transmission density of at least 2.5 is desired.
One way to increase the density of a transferred image is to merely increase the amount of dye in the dye-donor element and also to increase the amount of power used to transfer the dye. However, this is costly in terms of material and power requirements. Moreover, it is difficult to coat higher amounts of dye in the dye-binder layer. Furthermore, increasing the power to the thermal head generally causes deformation of the receiving sheet.
Another way to increase the density of a transferred image is to lower the amount of binder in the dye-donor element, thereby lowering the path length of the diffusing dye and increasing the dye transfer efficiency. However, when the content of dye in the dye-binder layer is enhanced, the dye tends to crystallize during storage of the dye-donor element. Moreover, the dye-donor element having an enhanced content of dye tends to stick to the receiving sheet during the printing operation.
Other ways to increase the density of the transferred image are to either find new dyes that have higher thermal dye efficiency or find materials that can be added to the dye-binder layer to increase the transfer efficiency in case transparencies are to be made. This would mean, however, that for making transparencies different dye-donor elements would be required than for making reflection prints. Such measure would result in increased manufacturing costs and inconvenience to the user.
In U.S. Pat. No. 4,833,124 a process has been described for increasing the density by printing twice or several times in register on one side of a receiving sheet. Unfortunately, this procedure suffers from several important disadvantages. It needs a considerable length of donor element. It is very time-consuming since it involves repeated passages of the receiving sheet along the thermal printing head. Moreover, only limited increases in density can be accomplished because the dye-image-receiving layer of the receiving sheet is saturated for the greater part by the dye transferred during the first passage and as a result of said dye saturation accepts far less dye during the next passage(s). Furthermore, during passage of the dye-image-receiving layer for the second time or subsequent times along the thermal printing head the already transferred dye partially migrates back to the dye layer. Thus, the density of the transferred dye image only increases in a limited way during the second and especially during further passages.
It would be desirable to provide a way to increase the density of transferred images in thermal dye transfer processes without suffering from the above-mentioned disadvantages.